Frank Oteri has a fine piece about Johnston in the NewMusicBox that embeds a fine recording by the Kepler Quartet playing some of Johnston's Tenth Quartet, but Oteri makes only anonymous reference to the Composers String Quartet's recording of Johnston's Second Quartet (c. 1967), which was the first recording of the piece. Just for the historical record, the Composers String Quartet predates the Kronos Quartet by a generation.
The original members of the Quartet, Matthew Raimondi, Anahid Ajemian, Bernard Zaslav, and Seymour Barab, along with their forward-thinking manager Gunther Schuller, are all still around. These are people who deserve a great deal of recognition. They are the ones who first championed the American string quartets written during the middle decades of the 20th century, and I hate to see only an anonymous reference to their contributions. They devoted years of work to rehearsing, premiering, and commissioning scads of difficult (but ultimately worthwhile) music.
Much of what they did was not commercially recorded, because only the most progressive record labels saw the value of mid-20th-century new music when it was new. Perhaps there are non-commercial concert recordings of the Composers Quartet playing "old" new 20th-century music an office or two, somewhere.